Tom, good point about a possible gap, but I have seen it up close and it definitely is a deep impression.
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Tom, good point about a possible gap, but I have seen it up close and it definitely is a deep impression.
Hey Maik, I think there may be crossed lines or a mis-understanding- I am referring to cutting M16 threads by hand in 316 stainless steel. This is a world away from doing the same in any low carbon steel like 'mild steel', 'bright steel' bar etc.
316 stainless has a pretty notorious reputation- even in industry for its ability to chew up and destroy cutting tools. Many workshops even try and avoid working with it and doing certain types of work as it requires very high grade cutting tools and careful handling in order to work effectively, and this of course adds to time and costs of the job.
We bought the taps (1st taper, 2nd taper + 3rd bottoming plug) and the wrench new from J&L specifically for the job. They were selected because the thread needed to be cut by hand and not on the lathe.
The torque needed was enough to twist the workpiece held in a large 3-jaw chuck bolted on the bench, and that was with copious amounts of liquid fluid intended purely for threading stainless steels. (God that stuff stank and makes me feel sick just thinking about it!)
The parts were stepped cylinders about 70 mm down to 50 mm diameter, that were previously turned and the blind hole bored 50 mm deep for the thread. Luckily, they were to be arc welded onto large 20mm thick angled plates of 316, so this was done first and then it was bolted rigid instead of gripping in the 3-jaw. This was the answer and prevented the slippage during thread cutting.
Stainless steels are very misunderstood alloys, and one unusual trait of some of the 316 group is they unusually work harden when machined too fast or when worked cold like during thread cutting.
Because they cannot be hardened by any of the normal thermal processes associated with treating steels, this characteristic is often overlooked and mis-understood. Mercury's comments about work hardening and plastic deformation were spot on, and although I wouldn't make a guess at to whether that's what happened to the Cleveland Fed paper clip, it's certainly what you often have to deal with, when working with 316 stainless.
Anyway, Ive never heard of anyone tapping threads into 316 stainless in any size and then saying its easy, or 'moderate' torque or 'momentum' as you described it on a thread like M16! Im thinking you must have some unearthly secret which you must tell the world! :(nerd):
Toms point was a good one, I'd started wondering about a possible gap and whether the paper clip was perhaps corroded onto the frame more than imprinted into it. But then Doug confirming he's seen it and it's clearly imprinted means we're back to square one again.
Doug I can see your point about the paper clip having a small amount of spring from the manufacturing process. Any form of clip would normally be a spring steel, but when you think about those traditional shaped clips they have very little or next to nothing in terms of a Spring action. It's just down to what little the thin wire has. When you bend one they normally just twist and stay in place. Perhaps there are many variations on them and some have been made from harder spring steel over the years. It would certainly explain things, until then this ones always going to baffle me and be a bit of a mystery me thinks :(nod):
Hey Huw :)
Okay i guess the steel i machined then never were 316. The stainless steel i have in my shop is used for knives and has a bad reputation for machining as well. But its still managable. V2A is also not friendly to cutting inserts and i never tried to cut a thread in that one. But mostly im machining mild steel (my whole vault door is made of it) and tool steel. But now i understand why you had so much trouble with cutting an M16... :(happy):
Yes the paper clips will not return if bent beyond their intended use. But they are not dead soft either. The door and frame on the other hand, obviously are softer. I haven't seen the infamous clip itself but the indentation is deep. The surface of the indentation itself will have seen the greater degree of work hardening than the paper clip. I agree it does seem strange but I am sure there were many factors considered in picking the choice of metal used in the door and frame castings, such as has been pointed out with hand filing. I haven't done any steel casting yet but in working brasses and bronze some are far better than others, regarding flow characteristics, gas holes, etc.
Here are a couple historic photos of the Cleveland Federal Reserve Vault:
Vestibule/Frame transport:
Attachment 16227
Main vault door opening on the left and Emergency vault door opening on the right:
Attachment 16228
I modified the crane hinge sketches to include all possible combinations (16) however, I have only found 4 of the crane hinge configurations in use.
Attachment 19123
aha -I should have seen that